Catalyst systems which catalyze the homo- or co-polymerization of unsaturated monomers have in the past gained great importance. The essential constituents of those catalyst systems (single site catalysts) are a transition metal complex as the active catalyst component and a co-catalyst component, preferably an alkylaluminoxane or the salt of a non-coordinating anion. As the transition metal component, preferably .pi.-complexes of groups 3 to 8 of the periodic system of the elements, especially (substituted) cyclopentadienyl complexes of group 4, as well as diazadiene derivatives of group 8, especially of nickel and palladium are used. Methylaluminoxane or (substituted) tetraaryl borates are preferably used as the co-catalyst component. Unsaturated monomers are preferably ethene, .alpha.-olefins, diolefins, vinyl or acrylic compounds, especially vinyl acetate, methyl methacrylate or acrylonitrile. Such systems, in contrast to the conventional Ziegler-Natta systems which are widely used, as well as being highly active and highly productive, also allow the product properties to be controlled in a specific manner in dependence on the components and reaction conditions used and make it possible to obtain novel polymer structures having very promising properties with regard to their commercial application.
A large number of publications relating to the preparation of specific polymers using the mentioned catalyst systems are known from the literature. In EP-B-129,368, for example, catalyst systems are described consisting of (substituted) cyclopentadienyl metal complexes with metals of groups 4 to 6 and methylaluminoxane, which permit the polymerization of olefins. Another catalyst system for the polymerization of olefins and acrylate monomers is disclosed in WO 96/23010, which describes the preparation of polyolefins using diazadiene complexes with metals of groups 4 to 8. The subject of EP-A-420,436 is hetero atom-substituted, bridged monocyclopentadienyl complexes with metals of group 4, which, by combination with aluminoxanes, catalyze the polymerization of olefins. EP-B-509,294 describes the homo- and co-polymerization of .alpha.-olefins with the aid of a catalyst system consisting of (substituted) monocyclopentadienyl complexes of chromium and non-coordinating anions.
Under polymerization conditions, the described catalyst systems produce polymers which are generally insoluble in the reaction medium. The (homogeneous) catalyst systems, which are soluble in the reaction medium, therefore form polymer deposits on the walls of the reactor, which make thermal control of the reaction difficult. A further disadvantage is that they are generally not suitable for controlling the morphology of the polymer particles.
The described disadvantages can be avoided by applying the above-mentioned active polymerization catalyst systems to support materials which are or have been rendered inert, such systems, thus, being rendered heterogeneous. Such supported catalyst systems have the advantage that they can be used in gas-phase processes and it is furthermore possible to specify the mentioned morphology of the polymer particles directly by the particle shape of the support. Furthermore, catalyst systems which have been rendered heterogeneous generally achieve their maximum activity at lower co-catalyst/catalyst ratios than when used in homogeneous form, as is shown, for example, by W. Kaminsky, et al. in J. Macromol. Sci., Rev. Macromol Chem. Phys. 1997, C37, 519-554.
Several possible methods of fixing polymerization catalyst systems, especially metallocenes, to supports are known. EP-A-206,794 describes heterogeneous catalyst systems obtained by adding the metallocene catalyst and the aluminoxane co-catalyst simultaneously or in succession to an inorganic support material. EP-B-323,716 and EP-B-367,503 describe the preparation of a heterogeneous catalyst system consisting of a metallocene catalyst and an aluminoxane co-catalyst, in which the aluminoxane component is formed in situ by reaction of a trialkylaluminium derivative with undehydrated silica gel and the active catalyst system is obtained by the subsequent addition of the metallocene component. EP-A-474,391 and EP-A-314,797, on the other hand, disclose heterogeneous catalyst systems characterized by the prior formation of the fixed co-catalyst component by reaction of an organoaluminum compound with the hydroxyl groups of the support material.
WO 96/00243 describes the supporting of metallocene catalysts and aluminoxane co-catalysts on porous support materials, wherein a solution of the metallocene and of the aluminoxane is formed and is brought into contact with the support material, and wherein the volume of the catalyst solution corresponds to from one to three times the pore volume of the support material used, so that the volume of the catalyst solution is not sufficient to form a slurry. WO 96/00245 describes supported catalyst systems which are prepared by bringing a metallocene and co-catalyst solution into contact with a porous support material, wherein the total volume of the metallocene and co-catalyst solution is less than the volume to form a slurry but at least greater than the pore volume of the support material used, and wherein, furthermore, methods in which the total volume of the metallocene and co-catalyst solution is less than the pore volume of the support material used are expressly excluded. Furthermore, in WO 96/14155, a process is described for supporting catalyst systems wherein the porous support is impregnated with an aluminoxane solution or a metallocene-aluminoxane solution and wherein the volume of that solution is less than twice the total pore volume of the support, less than the volume required to form a slurry or suspension and, moreover, greater than or equal to the total pore volume of the support. The methods described in WO 96/00243, WO 96/00245 and WO 96/14155 have the disadvantage that the supported catalysts prepared in accordance with that teaching are not obtained in the form of free-flowing powders, which is disadvantageous in commercial processes.
U.S. Pat. No. 5,688,734 describes a process for the preparation of prepolymerized supported catalyst systems which are obtained by first impregnating a porous support with the reaction product of metallocene and aluminoxane in hydrocarbon solvents, then drying the reaction product and subsequently adding aliphatic hydrocarbon solvents, and which are used for prepolymerization with gaseous monomers. The volume of the metallocene-aluminoxane-hydrocarbon mixture is to be less than or equal to the total pore volume of the system. Disadvantages of that process are the comparatively large number of process steps required to obtain the usable prepolymerized catalyst system, and the fact that the ionic catalyst system produced on the support is in heterogeneous form on account of its poor solubility in the aliphatic hydrocarbons.
In summary, all the immobilization methods described in the prior art satisfy the requirements made of an industrially usable catalyst system with regard to control of particle morphology and ready separability, but they are characterized by poorer activity in comparison with homogeneous systems or by the fact that, as mentioned, they cannot be used in the form of free-flowing powders in suitable processes.